A hydrostatic transmission is typically used to drive an output at a steplessly variable and normally reversible rate from a prime mover whose output rotation rate is normally one-way and of limited variability. Such an arrangement is particularly useful with, for instance, a diesel engine which only runs efficiently over a very limited speed range, the reason why diesel equipment, when equipped with a standard transmission, must have a great many different gears so as to maximize fuel efficiency.
Such systems are typically relatively complex and require that the engine be adjusted by the operator independently of the desired operating speed.